This invention relates to a new compound having utility as a luminescent material, and more particularly relates to a hexagonal hafnium, zirconium phosphate compound luminescent under x-ray, ultraviolet, etc. excitation, and also relates to a method for producing large single crystalline particles of such material, and to x-ray intensifying screens incorporating such material.
A problem associated with x-ray intensifying screens when used with conventional double emulsion x-ray film packs in so-called "cross-over". Cross-over is a loss of resolution on the exposed film attributed to the phenomenon of a photon in the visible spectrum passing through the emulsion on one side of the film pack, through the film base, and exposing the emulsion on the far side. Since the photons are transmitted in all directions from the phosphor on the intensifying screen, the image can become less sharp as a result of the far side exposure. Attempts have been made in the past to incorporate dyes into the film base which would render the base opaque to visible light, thereby eliminating cross-over. Such attempts have not been completely satisfactory.
Currently, x-ray film packs use plastic supports such as polycarbonates, polystyrenes, polyesters and the like as film bases, which while being opaque to ultraviolet light, are transparent to visible radiation. Since the film emulsions are sensitive to ultraviolet light, finding an x-ray phosphor having sufficient brightness (speed) and only emitting in the ultraviolet region would result in a sharp film image by the substantial elimination of cross-over.
Self-activated hafnium pyrophosphate, zirconium pyrophosphate, and mixed hafnium, zirconium pyrophosphate luminescent materials, which emit in the lower ultraviolet region of the electromagnetic spectrum, are known. For example, U.S. Pat. No. 2,770,749 issued to A. Bril et al. discloses a zirconium pyrophosphate phosphor. I. Shidlovsky et al., in "Luminescence of Self-activated Hafnium Pyrophosphate", Abstract #95 of the May, 1974 Meeting of the Electrochemical Society, disclose that hafnium pyrophosphate emits in the ultraviolet region more efficiently than zirconium pyrophosphate when excited by photoluminescence and cathodoluminescence. In U.S. Pat. No. 3,941,715, Shidlovsky discloses mixed (Hf,Zr) P.sub.2 O.sub.7 phosphors which emit in the lower ultraviolet region (below about 3,000 Angstroms) when excited by x-rays, cathode rays or visible radiation.
Hafnium phosphate luminescent materials having the formula Hf.sub.3 (PO.sub.4).sub.4 and containing activator elements such as Cu and Eu are disclosed in U.S. Pat. Nos. 3,905,911 and 3,905,912 to J. E. Mathers, and assigned to the present assignee. However, upon excitation by x-rays these materials emit in the visible portion of the spectrum, and are therefore of no utility for x-ray intensifying screen applications requiring ultraviolet emissions.